Gas sensing device

ABSTRACT

A device for sensing a gas comprises a plastics housing ( 106, 107 ) moulded in situ around at least one portion of a conducting lead frame ( 100 ), the housing defining an enclosure ( 113 ) and being provided with means for enabling gas flow into the enclosure. A gas sensitive element ( 114 ) within the enclosure ( 113 ) is mounted to the conducting lead frame ( 100 ). The conducting lead frame ( 100 ) comprises connection leads which are accessible through, and at least partially encapsulated by, the wall of the housing.

This invention relates to a device for sensing a gas, in particular aflammable gas, in air.

Catalytic oxidation is a well established and often used method fordetecting flammable gases in which an element is heated in order tooxidise any flammable gas present. During oxidation, heat is evolvedwhich causes the temperature of the element to rise and results in anincrease in its electrical resistance which may be detected to indicatethe presence of a flammable gas. Typically, the element comprises afilament of thin metal wire on which a porous bead is formed whichincludes a catalyst. Commonly, one such element is used in conjunctionwith a second, of similar construction but catalytically inactive, in aWheatstone bridge circuit. Thus the inert element may act as a control,compensating for variations in ambient temperature. To ensure combustionof the flammable gas, the elements must be operated at an elevatedtemperature, of the order of 500° C.

To ensure safe operation of such a device, the elements must be enclosedwithin a housing which prevents gas outside the device being ignited bythe gas undergoing combustion inside. However, in order for the deviceto function, gas must be able to flow into the enclosure. This isachieved by the inclusion of a flame arrestor in the housing, such as ametal mesh or a sinter element, through which gas may enter the deviceyet an ignition source may not escape.

Certain regulations must be met by the housing in order for it to becertified as flameproof and capable of withstanding the rapid andsignificant changes which can arise on combustion of the flammable gasinside the device. These changes may be in terms of pressure,temperature, chemical composition etc. All in all, the device must notallow ignition of the external gas mixture, irrespective of theconditions inside the device. This is achieved by a number ofprecautions, including:

the use of a flame arrestor to conduct heat away sufficiently rapidlythat a flame cannot propagate through this component;

ensuring that the strength of the housing materials and design issufficient to prevent rupturing as a result of extreme conditionsinside; and

confirming that any gas expelled from the sensor has insufficient energyto cause an external ignition, for example by limiting its exit rate andtemperature.

In virtually all cases, it is necessary to provide the device withcomponents which must pass through the housing wall, for exampleelectrical connectors. This results in gaps between the housing and theprotruding component(s) which could allow ignited gas to escape. In sucha situation, the device must be provided with one or more layers ofpotting compound, cement or other encapsulant which serve to seal thegaps and complete the flameproof enclosure. At present, there areregulations which specify the minimum thickness of such layers. In theUK, the potting compound typically has a minimum thickness of 3 mm.

The result is a final device size which is significantly larger than thevolume required by the operational components alone. It would beadvantageous to reduce the size of the final product, and in particularreduce it from a 3 dimensional object to a substantially flat device.This would lead to possible new uses of such a gas sensor, such as aclip-on badge sensor which could be worn by workers in a potentiallydangerous industrial situation, for example.

The majority of known flameproof housings are cast from metal andrequire several millimeters of potting compound and flame arrestormaterial, as well as impact protection for the flame arrestor in orderfully to meet the relevant safety and performance standards.EP-A-0667519 achieves a small reduction in device size by mounting thegas sensitive elements onto a track carrying substrate such that theymay be contained within the thickness of a typical PCB. However, theremainder of the device is largely conventional and hence the final sizeis not significantly reduced.

GB-B-2328508 describes a method of joining the flame arrestor to thehousing which overcomes the need for precision machining and acceleratesthe fixing operation whilst producing a join which is certifiablyflameproof. The invention makes use of a plastics housing which ismoulded in situ around the flame arrestor, which is in the form of ametal sinter material. This does not however address the issue of devicesize.

In accordance with a first aspect of the present invention a device forsensing a gas comprises at least one gas sensitive element containedwithin a flameproof, plastics housing supporting a flame arrestor whichenables gas to flow into the interior of the housing, and the gassensitive element(s) being connected to conducting leads which areaccessible through, and are at least partially encapsulated by, the wallof the housing, the encapsulating wall having sufficient thickness suchthat the housing will not allow the propagation of an ignition sourcefrom within the device to the ambient atmosphere, under workingconditions.

This first aspect of the invention further provides a method ofmanufacturing a device for sensing a gas, the method comprising mouldinga plastics housing in situ directly around a set of conducting leads,mounting at least one gas sensitive element inside the housing andconnecting it or them to the conducting leads which are accessiblethrough, and at least partially encapsulated by, the wall of thehousing, the encapsulating wall having sufficient thickness that thehousing will not allow the propagation of an ignition source from withinthe device to the ambient atmosphere, under working conditions, andsecuring a flame arrestor to the housing which completes the flameproofenclosure yet enables gas to flow into the interior.

The present invention eliminates the requirement for a layer of pottingcompound. By encapsulating a sufficient length of the conducting leadsin the plastic wall (by which we mean that there is intimate contactbetween the wall and the conducting leads), there is no gap at themetal/plastic interface which could allow ignited gas or another suchignition source to escape the flameproof enclosure. Therefore asignificant size reduction is possible and a substantially flat devicemay be constructed. In particular, by enabling the use of electricallyinsulating (plastic) materials for the enclosure around the electricalconnectors, a whole new range of design freedoms are made possible.Previous inventions such as GB-B-2328508 have not recognised suchmaterials as suitable to fulfil this role. One key benefit of thepresent invention is the ability to arrange that encapsulation occurs inthe horizontal plane rather than in a vertical direction as wouldusually be the case for a practical, compact, potted device.

Preferably, the plastics housing is fabricated by moulding in situ theplastics material directly around the conducting leads. This method notonly produces a flameproof seal between the conducting leads and thehousing but also simplifies the assembly process, being less expensiveand more easily controllable than traditional methods of incorporatingelectrical connections through the housing wall, which involve potting.Potting requires liquid handling and pouring, and the potting materialmay undergo contraction during solidification which must be compensatedfor by making the layer thicker than would otherwise be necessary. It istherefore highly advantageous to form a flameproof seal in one mouldingstep, without the need for potting.

Preferably, the device further comprises at least one filter in order toremove contaminants from the gas flow into the device. Certainsubstances may have a detrimental effect on the operation of the deviceshould they reach the gas sensitive elements, and should therefore beremoved by appropriate filters. Generally, at least one filter isprovided which removes hydrogen sulphide from the gas flow into thedevice. Typically, at least one of the filter(s) are inboard of theflame arrestor. This provides some degree of protection and holds themin place without the need for further fixings. However filters may alsobe located outboard of the flame arrestor, possibly held in place by atleast one clip.

The device preferably further comprises means for protecting one or moreof the gas sensitive element (s) from shock damage. This aims tominimise damage should the device suffer mechanical shock. Generally,the device further comprises means for insulating the gas sensitiveelement(s) and electrical connections, either in terms of electricalinsulation or heat insulation, or both. Preferably, the protectingand/or insulating means comprise at least one layer of shock absorbentand insulating material. The two functions may be carried out by thesame material, provided it is inert, has suitable mechanical propertiesand low heat conductance. Typically, the shock absorbent and/orinsulating material is glass wool.

Preferably, the flame arrestor is provided by a metal mesh. This is anadvantageous alternative to using a sintered metal powder, since anequivalent flameproof standard may be achieved using a thinner section.Generally, the flame arrestor is joined to the plastics housing by aprocess of thermal bonding around its perimeter. This is achieved byapplying pressure to the periphery via a hot workpiece, and results in aflameproof bond between the flame arrestor and the housing, by means ofplastic flow into the voids in the metal mesh.

Typically, the device further comprises a compensating element, whichbehaves similarly to the gas sensitive element except in its response tothe gas. When the device is connected to detector circuitry, the twoelements form part of a Wheatstone bridge circuit which provides asignal proportional to the gas concentration. Generally, the gassensitive element comprises a catalytic bead, such as a pellistor. Thecompensating element is catalytically inactive. However, the gassensitive element could also comprise a micromachined or planarpellistor or other types of heated gas sensor, for example semiconductorsensors or those which rely on thermal conductivity to detect gas.

Preferably, each gas sensitive element and/or compensating element ispositioned at least partly within a recess in an interior wall of thehousing. Also preferably, each recess also contains means for theprotection and insulation of the gas sensitive element positioned atleast partly inside it. This provides further protection against impactdamage and reduces heat loss from the gas sensitive elements.

The thickness of the portion of the housing wall through which theconducting leads extend is usually substantially at least 6 mm. Thelength of encapsulation of the conducting leads will be chosen to meetthe safety requirements for flameproof certification, which may changein due course as standards evolve.

Although the encapsulation length may be longer than with conventionalpotted devices, there is greatly increased design freedom. For example,typically, the flame arrestor is located above the gas sensitiveelement(s), the conducting leads extending out through a side wall ofthe housing. This has the advantage of making it possible to construct asubstantially flat device.

Preferably, the conducting leads are coupled with respective contactslocated in an integral extension of the housing. This configurationenables the device to be connected to other electrical components in avariety of ways, as may be chosen to suit each respective application.

Conveniently, the conducting leads are provided by a conducting leadframe fabricated prior to encapsulation by the plastics housing. By“lead frame”, we refer to a conducting portion of the device, ratherthan to a frame made out of lead (Pb). This enables straightforwardmoulding of the plastics housing directly around the metal, and is moreconvenient than incorporating more than one component during themoulding process.

Typically, the conducting leads will form pads flush or sub flush to thehousing although they could extend or protrude through the housing.

In the preferred example, the housing is formed by moulding the plasticsmaterial around the lead frame and subsequently mounting the othercomponents. However in some situations it may be more appropriate tomould the housing around more of the components, in situ. For example,the gas sensitive element(s) could be connected to the lead frame beforethe housing is moulded.

Preferably, the device further comprises means for retaining componentslocated outboard of the flame arrestor. Typically, the retaining meansis provided by a bezel which fastens mechanically to the housing. Thebezel also provides some degree of mechanical protection for the flamearrestor and filter(s).

In accordance with a second aspect of the invention, a device forsensing a gas comprises a plastics housing moulded in situ around atleast one portion of a conducting lead frame, the housing defining anenclosure and being provided with means for enabling gas flow into theenclosure, and at least one gas sensitive element within the enclosuremounted to the conducting lead frame, wherein the conducting lead framecomprises connection leads which are accessible through, and are atleast partially encapsulated by, the wall of the housing.

This second aspect of the invention further provides a method ofmanufacturing a device for sensing a gas, the method comprising mouldinga plastics housing in situ around at least one portion of a conductinglead frame such that the housing defines an enclosure, providing thehousing with means for enabling gas flow into the enclosure, mounting atleast one gas sensitive element inside the enclosure and connecting itto the conducting lead frame, and providing the conducting lead framewith connection leads which are accessible through, and at leastpartially encapsulated by, the wall of the housing.

By overmoulding the lead frame, the housing acts not only as anenclosure and support for the sensor components and electricalconnections but also as insulating means between conducting regions.This makes it possible to reduce the size of the sensor envelope and cansimplify the manufacturing process by reducing the number of assemblysteps required.

Generally, portions of the lead frame are left uncovered by the plasticshousing. This provides suitable connection points to which electroniccomponents may be mounted. Typically, the device further comprises anelectronic component mounted onto at least some of the portions of theconducting lead frame not covered by the plastics housing. Preferably,the electronic component is a memory component, but it could also oralternatively comprise components such as analogue to digitalconverters, amplifiers or microprocessors mounted in a similar manner.If desired, several electronic components could be connected to theconducting lead frame, the lead frame fulfilling the function of acircuit board in creating the required interconnections. In the case ofa memory component, the memory component is preferably an EEPROMalthough it could be many other types of memory chip. The memorycomponent generally stores data relating to the gas sensitive element,but it could also hold any other information of value to the sensoruser, such as date of manufacture, calibration data and temperaturecompensation coefficients.

By attaching an EEPROM or other component directly to the lead frame, itbecomes an integral part of the sensor construction, rather than anadd-on component housed in an extension to the sensor envelope aspreviously known. In this respect, the lead frame acts not only as asensor support and means of electrically connecting power and signalinputs to and from the sensor, but also as means of electricallyconnecting power to the EEPROM and also transmission of signals into andout from the EEPROM. Further, the lead frame provides the basicstructure of the connector used to interface the completed sensor systemto external circuitry.

Conveniently, the device further comprises a cap which covers at leastsome of the portions of the conducting lead frame not covered by theplastics housing. Such a cap could be a separate moulding which snapsonto the sensor housing or the cap could be moulded directly onto thedevice during an additional moulding step. The cap provides protectionfor the exposed parts of the lead frame and any electronic componentsattached thereto.

Preferably, the lead frame is overmoulded by the plastics housing in twosteps rather than one, so that the plastics housing comprises at leastan inner portion and an outer portion, the outer portion being mouldedaround the inner portion. Alternatively, the two steps could be combinedinto one. However, by forming the housing in two steps, it is possibleto tune the properties of the plastics materials used to suit the sensorand the application. For example, different plastics may be used to formthe outer and inner parts of the housing. Also, in practice, it isdifficult to mould a relatively large volume of material around the leadframe in a single step.

It is preferable that the or each gas sensitive element is asemiconductor gas sensor although other forms of sensor might beemployed such as those based on conductive polymers or ion selective FETstructures. If a semiconductor sensor is employed, preferably the oreach semiconductor gas sensor comprises a p-type mixed metal oxidesemiconducting material of the first, second and/or third ordertransition metal series, wherein the semiconductor gas sensor isresponsive to a change in concentration of carbon monoxide in thesurrounding atmosphere.

It is further envisaged that certain features of sensors according tothe first aspect of the invention may be incorporated into a sensoraccording the second aspect of the invention, or vice versa. Forinstance, the overmoulded lead frame can be designed to provide aflameproof seal between the conductive frame and the housing. Byproviding the sensor with a flame arrestor, the enclosure may bearranged to be flameproof and the device used to detect combustiblegases by means of a pellistor or other suitable gas sensitive element.

Examples of a gas sensors incorporating a device for sensing a gas inaccordance with the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 shows an exploded view of a gas sensing device in accordance witha first aspect of the invention;

FIG. 2 is a schematic plan view of the housing, gas sensitive elementsand conducting leads as shown in FIG. 1 but with the conducting leadsextending out of the housing;

FIG. 2A is a schematic underneath plan showing projections of theuppermost features of the devices as dashed lines, the location of thelead frame in solid lines and shortest distances along the conductingpath, though the wall of the housing in bold;

FIG. 3 is an underneath plan of the device as shown in FIG. 1;

FIG. 4 is a section taken on the line A-A in FIG. 2, but illustrates acomplete device with the exception of the metal bezel 14;

FIG. 5 comprises FIGS. 5 a and 5 b which show, respectively, a plan anda side view of a conducting lead frame before the housing has beenmoulded around it;

FIG. 6 is a circuit diagram of a detecting circuit attached to thedevice;

FIG. 7 is a perspective view of an example of a lead frame which may beemployed in a gas sensing device in accordance with a second aspect ofthe invention;

FIG. 8 is a perspective view of the lead frame shown in FIG. 7overmoulded with a first portion of a plastics housing;

FIG. 9 is a perspective view of the lead frame shown in FIG. 7overmoulded by both an inner portion of the plastics housing and anouter portion of the plastics housing;

FIG. 10 is a perspective view of the assembly shown in FIG. 9 with anelectronic component shown connected to the lead frame;

FIG. 11 is a perspective view of the completed gas sensor device;

FIG. 12 is an underneath perspective view of the overmoulded lead frame;and,

FIG. 13 is an underneath perspective view of the overmoulded lead framewith the gas sensing element in place.

The gas sensing device shown in FIGS. 1 to 4 comprises a plasticshousing 1, made from a material such as PEI (polyetherimide), PPS(polyphenylsulphide) or PTFE for example, moulded around a metal leadframe 2, which is shown in more detail in FIG. 5. The housing includes awall 10 which surrounds a cavity, at the base of which is asubstantially square recess 12 which itself is surrounded by a raised,ridged shelf 11 which is substantially circular. In the floor of therecess 12 are two further recesses 13 containing glass wool 9 which actsas a shock absorber and insulator.

Mounted in the housing 1 are two conventional pellistor gas sensingelements: a detector 4 and a compensator 5. Each element comprises ametal filament encased by a porous bead, the detector element 4including also a catalyst which may be chosen according to the gas whichis to be detected. The elements 4 and 5 are connected to the lead frame2 by means of conducting cement or welding, for example. The metal leadsare encapsulated by the wall of the housing 10 and exit the housing bodyas tabs 22-24 as shown in FIGS. 2 and 3, although in other examples theymay not extend through the apertures 18-20 as shown in FIG. 1.

A second layer of glass wool 8 is located above the gas sensitiveelements 4 and 5, inside the recess 12. A hydrogen sulphide filter 7rests on the housing floor 11 a above the recess 12. The H₂S filter 7 istypically fabricated using a paper or glass wool filter impregnated withlead acetate and as such it is desirable that it is inboard of the flamearrestor 3 to prevent users from coming into contact with it. Anenclosure is created by the joining of the metal mesh flame arrestor 3to the top of the shelf 11, which is sufficiently wide to ensure thatthe resulting joint is at least 1.25 mm wide. The mesh 3 allows thepassage of gas into the cavity yet acts as a flame arrestor and thusrenders the enclosure flameproof. A second, silica filter 6 is locatedon the outboard side of the flame arrestor 3 and comprises a glass fibredisc coated with 25% Si. Its purpose is to absorb chronic, irreversiblepellistor catalyst poisons such as the silicone HMDS. The two filters 6and 7 together remove contaminants from the gas flow into the device. Itis also envisaged that plural filters such as 6 and 7 could be combinedinto a single, multi-purpose filter, although this is not shown in thedrawings.

A metal bezel 14 clips onto the housing 1 in order to hold componentssuch as filter 6 in place and to provide the device with protection. Thebezel 14 is provided with a number of holes 16 which enable the bezel 14to be fastened to the housing 1 by means of a corresponding number ofbarbs 17 on the housing exterior. The bezel 14 also incorporates one ormore holes 15 through which gas may enter the device.

The features of the housing 1 are shown in plan view in FIG. 2. Thedashed lines indicate the position of the lead frame 2. The gassensitive elements 4 and 5 are connected to the lead frame 2 whichemerges from the housing as leads 22-24, through apertures 18-20. Atleast 6 mm of each conducting lead is encapsulated by the wall of thehousing 1. FIG. 2A illustrates the shortest path on each of the threeconducting lines which pass through the housing to connect the gassensitive elements inside the flameproof enclosure to the detectingcircuit outside. It is clear that the path marked “Y” is the shortestpath and it is this path therefore which requires most scrutiny by thecertifying authorities. Generally path “Y” must be at least 6 mm long.The lead frame 2 also comprises a trimming resistor 21 which isconnected between the leads 23 and 24 and is present to compensate fordifferences in performance of the two elements 4 and 5.

FIG. 3 shows that the same leads 22-24 may be also accessed from beneaththe device, through apertures 25-27 respectively, which are formed in anintegral, lateral extension of the housing 1. This may be useful, forexample, when mounting the device onto a printed circuit board (PCB).Again, the lead frame 2 is represented inside the housing by dashedlines. The circles L, seen also in FIGS. 1 and 2 are a result of the useof lead frame locating pins during the moulding process.

The arrangement of the layers can be seen most clearly in FIG. 4. Thegas sensitive elements 4 and 5 are shown to be sandwiched between layersof glass wool 8 and 9 which provide protection from mechanical shock,electrical insulation and insulation from heat loss. In a typicalarrangement, the depth of the recess 13 over which the element 4 or 5 islocated is approximately 0.5 mm and its diameter 2 mm. The recess 12above the gas sensitive elements is generally about 1 mm deep and 5 mmby 5 mm in area. The device has a total height of approximately 4 mm.

The all-metal lead frame 2 is shown in more detail in FIG. 5, prior toits encapsulation by the plastics housing 1. Once the housing 1 has beenmoulded around the lead frame 2, the metal is cut along line 34 toseparate the conducting leads. This line need not coincide with theouter edge of the plastics housing and in particular conducting leadsmay be left protruding from the housing, as shown in FIGS. 2 and 3. Itis envisaged that the lead frame is constructed from beryllium copper,with a hard acid gold plating layer, substantially 0.5 microns inthickness, over electroless nickel. However, any number of othervariants could in principle satisfy the same requirements of goodmechanical stability, resistance to degradation caused by aggressiveoperational environments and good electrical conductivity. It would alsobe possible to have a lead frame 2 in the form of a pre-mouldedsubassembly, in which the joining part 35 is an insulating material andmay therefore remain attached, for example to assist in placing thedevice into an instrument. As shown in FIG. 5 b, the lead frame 2 neednot be flat. In the example shown, most of the interior portion of thelead frame 2 is raised slightly with respect to the exterior parts.

The detector element 4 is connected between points 28 and 29 on the leadframe 2, and the compensator element 5 between points 30 and 31 by meansof conducting cement or welding, for example. The trimming resistor 21joins points 32 and 33. Layers 8 and 9 of glass wool are packed aboveand below the elements 4 and 5 and a filter 7 is positioned on thehousing floor, on top of the glass wool 8. The flame arrestor 3 isjoined around its perimeter to the top of the shelf 11 of the housing 1by means of a thermal bonding process such as heat staking. Filter 6 isthen located on the outboard side of the flame arrestor, and held inplace by the metal bezel 14, which is mechanically fastened to thehousing by means of barbs 17 and holes 16. In use, gas passes though thebezel 14, the flame arrestor 3, both filters 6 and 7 and at least onelayer of glass wool 8 to reach the detecting element 4.

FIG. 6 is a circuit diagram to illustrate a Wheatstone bridge circuitwhich incorporates the two elements 4 and 5. As described above, theelements are coupled to leads 22-24, with a trimming resistor 21 betweenleads 23 and 24. The lead 24 forms one output 36 of the circuitdirectly. Leads 22 and 23 are joined to resistors R1 and R2 at points 37and 38 respectively. Resistors R1 and R2 are connected at point 39 to avariable resistor 40. Point 39 also provides the second output 41. DCpower is supplied to the circuit at 42, and is used to heat the elements4 and 5 to their working temperature of approximately 500° C. as well asto power the circuit. However, it should be noted that the sensors couldalso be operated in circuits other than the Wheatstone bridge describedabove.

An example of a gas sensing device in accordance with a second aspect ofthe invention will now be described with reference to FIGS. 7 to 13. Thesensing device consists of a conductive lead frame 100, partiallyencapsulated by a plastics housing 106 and 107, a gas sensitive elementconnected to the conducting lead frame 100 and an electronic component110 also connected to the conducting lead frame 100.

A conducting lead frame 100 is shown in FIG. 7. This component performsa number of functions. In particular, it provides physical support forthe sensor, electrical connections to and from the sensor, electricalconnections to and from any additional components which may beincorporated within the gas sensing device, interconnections between thesensor and the additional electronic components and the basic structureof the electrical connector used to interface the completed device tothe remainder of the operating circuitry.

The lead frame 100 may be constructed from a range of conductivematerials, typically metals such as copper or steel. Nickel-coatedphosphor bronze is particularly well suited to the application. The leadframe 100 may be fabricated in a number of ways, for example by aphotolithography and etching sequence, or by a series of stamping andbending processes (progressive forming). The detailed shape of thestructure is dictated by not only the requirements of the sensor andadditional component circuitry but also the eventual physical envelopeof the completed assembly.

Four pad areas 103 are provided on the lead frame 100 to allowconnection to the gas sensitive element itself. Tabs 104 are formed intoan array to facilitate direct connection of integrated circuit ordiscreet electronic components. The lead frame 100 illustrated in theFigures accepts an 8-pin package, but larger or smaller numbers ofconnections may be envisaged. It will be noted that some of tabs 104 arecommoned together; others are connected to one or more of the sensorpads. In one case, a direct link between the two tabs across theelectronic component is shown. Several tabs 104 are formed as extensionsof connector pins 105 which act as the basis of the external connector.

In this initial form, the lead frame is provided with sacrificial parts101 and 102 which greatly assist in handling the components duringsubsequent fabrication processes but which may be removed at appropriatestages by cutting or snapping along lines A and B which are providedwith weakening grooves for this purpose.

The sensor housing 106 and 107 is formed by means of injection mouldingwith the lead frame in situ. In other words, the lead frame ispositioned inside the mould during the injection moulding process. Thehousing is fabricated from thermoplastic materials such as polyphenylenesulphide (PPS) or liquid crystal polymer (LCP), although a range of suchmaterials is available. A two-stage moulding process is used in whicheither the same or different polymeric materials may be used in eachstage. This selection is largely dictated by the particular propertiesdemanded of the finished component. For example, when using heatedsensors, it may be that the region immediately surrounding the gassensing element is required to have very stable properties at hightemperature. In contrast, the external casing may need to offerresistance against a number of contaminants. Suitable polymericmaterials may be employed for each location. Clearly, the plastic mustbe an electrical insulator to prevent short circuiting the lead frame100.

In a first moulding step, a thermoplastic material is injection mouldedaround the lead frame 100 in order to form an inner portion 106 of thesensor housing. It should be noted that this first in situ moulding stepleaves pad areas 103, tabs 104 and the ends of connector pins 105exposed. In its unsupported state, the lead frame is fragile andflexible since the component is intended to be fully encapsulated in thefinal device. Thicker and hence more expensive lead frames could be usedto partially alleviate this problem, but in applications demanding verylow cost, this is not an attractive option. Therefore, an importantfeature of this first moulding step is that it provides strength and soeases handling difficulties during the remaining processing stages. Oncethe initial moulding has been completed, the sacrificial extensions 101and 102 may be removed, although in some circumstances, it may behelpful to leave either or both extensions in place until the subsequentmoulding stage is complete.

FIG. 9 shows the gas sensing device after a second moulding step, inwhich the assembly as shown in FIG. 8 is overmoulded by a second volumeof thermoplastic material. As discussed above, this material may be thesame or different to that used in the first moulding step. The secondthermoplastic material forms an outer portion 107 of the sensor housing.Like the inner portion 106, the outer portion 107 may be formed by insitu injection moulding. Two separate injection moulding tools may beused for the two stages, or a single tool allowing two or moulding shotsmight be used instead. As shown in FIG. 9, the outer portion 107 of thehousing still leaves the pads 103 and tabs 104 exposed for componentattachment. The second moulding step also forms the remainder of theconnector shroud 108 and circular features 109 which are used to attachthe completed device to the required apparatus.

It would be possible to overmould the lead frame 100 in a single step toproduce the whole casing. However, this would not allow the materialproperties to be varied and could damage the lead frame. A limitedamount of material can be moulded around such a flexible component asthe lead frame 100 in a single step without causing significantdistortion.

An electronic component 110, such as an EEPROM, is attached to theexposed lead frame at tabs 104 via component legs 111. These arepreferably joined by soldering, and a re-flow process is advantageous inthis context. However, conducting adhesive or other known methods can beused to secure and make electrical connection to component 110 (FIG.10). In order to complete this part of the assembly, a pre-moulded top112, shown in FIG. 11, is attached to housing 107 either by snap-fitmethods, or glueing, welding or other bonding means. If required, top112 can be made in a detachable form to allow component 110 to bechanged, but this is unlikely to be cost effective in low-priceapplications. Top 112 could be overmoulded in an additional mouldingstep, although the effect of the high temperatures upon the electroniccomponents 110 would need to be ascertained.

The underside of the assembly is shown in FIG. 12. The housing 107encloses a volume 113 into which a gas sensitive element may be fitted.Pad areas 103 of the conducting lead frame 100 are exposed within theenclosure, allowing a gas sensitive element 114 to be inserted into thecavity and electrically connected via leads 115 using soldering orconducting adhesive, for example (FIG. 13). In the example shown, sensor114 is a planar device employing a semiconducting material whoseelectrical properties vary in response to the composition of thesurrounding gas atmosphere. Particularly, a p-type mixed metal oxidesemiconducting material of the first, second and/or third ordertransition metal series may be selected, which is responsive to a changein concentration of carbon monoxide in the surrounding atmosphere andalso to a change in the concentration of oxygen in the surroundingatmosphere. For example, the sensor 114 shown could comprise chromiumtitanium oxide, but other forms of sensor might be employed, such asthose based on conducting polymers or ion selective FET structures.

After attachment of the sensor 114, the enclosure may be sealed ifrequired by a mesh, sinter, cap, membrane or other means allowingingress of gas to the enclosed volume. This may advantageously bemounted on lip 116 by a suitable method such as heat staking, welding orglueing, depending on the intended application. Filters may also beincorporated in-board or out-board of the seal, and retained by the sealor another closure component.

In this example, the electrical component 110 is shown as a memory chipwhich might contain a variety of information useful to the sensoroperator, such as calibration data or data relating to the target gasspecies. It is envisaged that the connections provided by pins 105 wouldallow this data to be interrogated by the user and this is also quitefeasible that rewriting of the data into user accessible regions of thememory could be undertaken, perhaps as a result of in-fieldcalibrations. Additionally, or instead of a memory chip such as aEEPROM, the lead frame could readily support a range of other componentssuch as analogue to digital converters, amplifiers or microprocessors.

The completed gas sensing device may be mounted to any desired surfaceby means of fixing points 109 (FIG. 9) with screws or nails for example.The device may be electrically connected to an external circuit viaconnection pins 10.5. The pins 105 are housed within a connector shroud108 which may be designed to fit with a connection plug.

1. A gas detector comprising: a metal connecting frame having aplurality of conductors extending between first and second ends, theplurality of conductors formed into connector pins on the first end witha first portion of the plurality of conductors on the second end formedinto a plurality of pads and a second, different portion of theplurality of conductors on the second end formed into an array of tabs;and a housing having a first portion formed of a first type plastic, thefirst portion, at least in part, surrounds the frame with elongatedportions of the frame extending through and encapsulated by portions ofthe first type of plastic with other portions of the frame accessible toprovide contacts exterior to the first portion, and with theencapsulated portion of the frame forming a flame proof seal with theencapsulating first portion with the housing having a second outerplastic portion, the second portion is molded over and encloses thefirst portion and non-contact portions of the frame, and is formed of adifferent plastic material with the second outer portion forming apartially enclosed molded plastic sensing region that supports theplurality of pads for a gas sensor with the second portion defining agas inflow port and with a side of the second portion opposite thepartially enclosed molded plastic sensing region supporting the array oftabs as a basis for an external connection to which electroniccomponents may be mounted on an exterior of the housing, wherein thesecond portion forms a shroud around the connector pins and wherein eachof the plurality of conductors extend from the connector pins into thehousing along a single plane, with the first portion of the plurality ofconductors extending to respective pads on opposing sides of thepartially enclosed molded plastic sensing region such that the flameproof seal is formed from at least a predetermined minimum length ofeach conductor of the first portion of conductors encapsulated inplastic and wherein at least one conductor of the first portion ofconductors follows a U-shaped path within the plastic to provide thepredetermined minimum length.
 2. A gas detector as in claim 1 where theencapsulated portion of the frame is at least 6 mm long.
 3. A gasdetector as in claim 1 where the sensing region is closed by a flamearrestor that covers the inflow port.
 4. A gas detector as in claim 3where the flame arrestor comprises a metal mesh.
 5. A gas detector as inclaim 4 where the metal mesh is, in part, bonded to the second, outer,portion by plastic filling voids in the mesh adjacent thereto.
 6. A gasdetector as in claim 4 which includes a filter located in the sensingregion between the gas sensor and the metal mesh.
 7. A gas detector asin claim 1 wherein the second, outer, portion carries an electronicstorage unit coupled to at least some of the contacts.